In recent years, for the transmission of a driving force alone in a specific direction, a rotation transmission device with a built-in one-way clutch has been used in automobile auxiliary machines such as an alternator, auxiliary machine driving devices and crankshafts of engines. The rotation transmission device with a built-in one-way clutch is an apparatus which includes an inner-diameter-side member, a cylindrical-shaped outer-diameter-side member located around the inner-diameter-side member concentrically with the inner-diameter-side member, rolling bearings located between the outer peripheral surface of the inner-diameter-side member and the inner peripheral surface of the outer-diameter-side member for supporting the inner-diameter-side member and the outer-diameter-side member while permitting relative rotation between the inner-diameter-side member and the outer-diameter-side member, and a one-way clutch adapted for transmitting only such a rotational power that rotates one of the outer-diameter-side member and the inner-diameter-side member relative to the other in a specified direction.
Such an alternator and the like now progress in performance and output and are used in a wide area including cold climate areas. As a consequence, the conditions under which the rotation transmission device with a built-in one-way clutch is used become severe. Namely, the rotation transmission device is required to operate at a higher revolution speed and a higher load and to achieve a desired performance under an extremely low temperature so as to withstand use in cold climate areas. In this circumstance, grease used in such a rotation transmission device with a built-in one-way clutch operated under severe conditions is desired to produce a high performance and to satisfy the following characteristics.
(i) The grease must provide satisfactory clutch engagement performance (intermeshing) at low temperatures. When an engine is started in an extremely cold area in winter, satisfactory clutch engagement performance (intermeshing) is demanded in order for an alternator or the like device to achieve smooth operation.(ii) The grease must be less apt to cause oil separation under high centrifugal force. Since auxiliary parts of automobiles such as alternator are operated at high revolution speed and used under high centrifugal force, the grease must be less apt to cause oil separation.
It is known that the grease performance at low temperatures may be improved by using a low viscosity base oil. Grease using a low viscosity base oil, however, generally causes oil separation, with the oil separation tendency increasing under high centrifugal force conditions. When, on the other hand, a high viscosity base oil is used, the grease performance at low temperatures is deteriorated though the oil separation tendency is reduced.
Namely, the good clutch engagement performance at low temperatures as described in (i) above and the reduction of oil separation under a high centrifugal force as described in (ii) above are generally opposing properties. It is, therefore, not easy to improve these properties at the same time.
As conventional greases for use in such a rotation transmission device with a built-in one-way clutch, there are disclosed grease in which an ether-based base oil such as an alkyl diphenyl ether is used (see, for example, Patent Documents 1 and 2), grease in which a base oil containing a polyol ester having a kinematic viscosity at 40° C. of 20 mm2/S or less is used (see, for example, Patent Document 3), grease in which a base oil such as a mineral oil, a poly-α-olefin oil or a polyol ester oil is used together with a thickener containing a diurea compound (see, for example, Patent Document 4), and grease in which a urea thickener is compounded into an ester-based or synthetic oil-based base oil having a pressure viscosity coefficient of 12 Pa−1 or more (see, for example, Patent Document 5).
The grease using an alkyl diphenyl ether as a base oil is not satisfactory with respect to low temperature properties, i.e. clutch engagement performance at low temperatures. The grease using a base oil containing a polyol ester is generally not fully satisfactory with respect to clutch engagement performance at low temperatures. The other base oils such as a poly-α-olefin oil have similar problems. Accordingly, there is a room for further improving the grease for use in a rotational transmission device having a built-in one-way clutch.
[Patent Document 1] Japanese Patent Application Publication No. 2006-162032
[Patent Document 2] Japanese Patent Application Publication No. H11-82688
[Patent Document 3] Japanese Patent Application Publication No. 2006-161827
[Patent Document 4] Japanese Patent Application Publication No. 2006-132619
[Patent Document 5] Japanese Patent Application Publication No. 2000-234638